Method for producing micro-optical components

ABSTRACT

A method for direct printing of micro-optical components (82) onto optical substrates (84) or active devices to create optical circuit elements as well as micro-optical components and systems, such as plano-convex circular, cylindrical or square lenslets, anamorphic lenslets, waveguides, couplers, mixers and switches and monolithic lenses deposited directly onto optical components such as diode lasers and optical fibers. The method provides a means for precisely depositing a wide range of materials in a wide variety of shapes for fabricating a full range of passive and active micro-optic devices.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSOREDRESEARCH AND DEVELOPMENT

This invention was made with U.S. Government support under Contract#F29601-94-C-0072 awarded by the United States Air Force. The U.S.government has certain rights to the invention.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.08/202,939 filed Feb. 28, 1994, now U.S. Pat. No. 5,498,444 which issuedon Mar. 12, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to a method for producingmicro-optical components. More particularly, it relates to a method fordispensing very small and precise amounts of optical materials forproducing micro-optical components.

2. Description of the Prior Art

The increasing demands in miniaturization and parallel processing ofoptoelectronic devices and the maturity of the process technologies inmicron-scale fabrication have pushed forward the development ofmicrolenses and other micro-optical components. According to the lensperformance principles utilized, the various types of microlensesdeveloped to date can be categorized as refractive lenses, diffractivelenses or mixed refractive/diffractive lenses. Refractive lenses bend orfocus a light beam by rules of geometric optics. Diffractive lensesalter the path of light based on Fourier optics. Mixedrefractive/diffractive lenses typically include refractive lenses havingthe surface thereof textured with diffracting patterns to correct forchromic aberrations.

Current techniques for fabricating micro-optic components include fiberend-surface etching; fiber tip etching and melting; lasermicromachining; fiber drawing; polymer island melting; localized UVradiated and heated photothermal glass; ion-beam etching of Si; ion-beametching of InP; chemical etching of InP; graded index finishingtechniques such as ion exchange in glass from molten salt, swelling thesurface of glass, chemical vapor deposition of SiH₄ and NO, and ion-beamsputtering of Si--O--N; and binary optics techniques including the useof 2-step Fresnel phase plates, 8-step Fresnel phase plates and blazedreflection grooves.

Small scale lenses offer performance advantages in faster optics andreduced aberrations. Individual lenses formed on the tips of opticalfibers and on diode lasers in addition to arrays of lenses, have beendemonstrated with reasonable performance characteristics, particularlyfor coupling into or out of fibers, detectors and diode lasers. Arraysof refractive lenslets have been used to provide efficient coupling ofarrays of energy sources to amplifier and detector arrays or to bundlesof optical fibers. In addition, telescopes, waveguides, couplers,relays, and spot generators incorporating small scale lenses have beeninvestigated. Materials which have been used for forming such smallscale lenses include glass, silicon, indium phosphide, polymersincluding amorphous Teflon® and plastics.

The production of such microlenses, however, has been severely limitedby the above-mentioned techniques. The present invention provides asolution to various prior art drawbacks and deficiencies including thelack of flexibility in micro-optics manufacture and the inability toprint micro-optical components directly onto optical substrates anddevices.

SUMMARY OF THE INVENTION

The present invention is directed to a method for direct printing ofmicro-optical components onto optical substrates or active devices tocreate optical circuit elements as well as micro-optical components andsystems, such as plano-convex circular, cylindrical or square lenslets,anamorphic lenslets, waveguides, couplers, mixers and switches andmonolithic lenses deposited directly onto optical components such asdiode lasers and optical fibers.

The method of the present invention involves the application of datadriven ink-jet droplet dispensing technology to form and placemicroscopic droplets of optical materials to create micro-opticalcomponents for use with diode lasers and amplifiers. The method of thepresent invention also involves the delivery of droplets of opticalmaterials directly onto the output facets of diode lasers to improvetheir performance and onto optical substrates to form arrays of highnumerical aperture microlenses for coupling arrays of diode lasers,amplifiers and optical fibers.

The method of the present invention further involves the placement oflenslets onto the ends of optical fibers and fabrication of activemicro-optical components, such as micro-lasers for high spectral purityor femtosecond applications.

The method of the present invention provides a means for preciselydepositing a wide range of materials in a wide variety of shapes forfabricating a full range of passive and active micro-optic devices.

The ink-jet printing methods of the present invention provide a meansfor data-driven fabrication of micro-optical elements such as refractivelenslet arrays, multimode waveguides and microlenses deposited onto thetips of optical fibers. The materials that can be used for microjetprinting of micro-optics include optical adhesives and index-tunedthermoplastic formulations. By varying such process parameters asnumbers and locations of deposited microdroplets, print head temperatureand orifice size, and target substrate temperature and surfacewettability, arrays of spherical and cylindrical plano-convexmicrolenses can be fabricated with dimensions ranging from 80 μm to 1 mmto precision levels of just a few microns, along with multimode channelwaveguides. An optical telescope system yielding optical performancedata such as lenslet f/#'s and far-field patterns along withbeam-steering agility data can be assembled from microlens arraysprinted according to the method of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the invention will become more apparentwith reference to the following detailed description of presentlypreferred embodiments thereof in connection with the accompanyingdrawings, wherein like reference numerals have been applied to likeelements, in which:

FIG. 1 is a simplified side elevational view of an ejection head of adevice for dispensing very small and precise amounts of opticalmaterials for producing micro-optical components according to the methodof the present invention;

FIG. 2 is a simplified side elevational view of an ejection head of adevice, with portions thereof cut away, for dispensing very small andprecise amounts of optical materials for producing micro-opticalcomponents according to the method of the present invention;

FIG. 3 is a schematic drawing of an apparatus constructed in accordancewith the present invention for dispensing optical materials;

FIG. 4 is a schematic drawing of an array of micro-lenses producedaccording to the present invention coupling the output of an array ofdiode lasers into optical fibers;

FIG. 5 is a schematic drawing of apparatus constructed in accordancewith the present invention for dispensing optical materials inconjunction with a vision system;

FIG. 6 is a schematic drawing of apparatus constructed in accordancewith the present invention for varying the size and shape ofmicro-optical components;

FIG. 7 is a schematic drawing of representative verticalthree-dimensional structures that may be produced according to themethod of the present invention;

FIG. 8 is a schematic drawing of representative verticalthree-dimensional structures that may be produced according to themethod of the present invention;

FIG. 9 is a schematic drawing of representative horizontalthree-dimensional structures that may be produced according to themethod of the present invention;

FIG. 10 is a schematic drawing of apparatus constructed in accordancewith the present invention for dispensing optical materials inconjunction with a source of radiation;

FIG. 11 is a schematic representation of the variation in plano-convexmicrolenslets created by microdroplets of fixed volume by variation ofcertain process parameters;

FIG. 12 is a plot of design curves for fabricating microlenses whichcollimate;

FIG. 13 is a schematic drawing of an apparatus constructed in accordancewith the present invention for dispensing optical material;

FIG. 14 is a plot of printed hemispherical lenslet focal length versuslenslet diameter;

FIG. 15 is an optical micrograph of a portion of a 50×50 array of 107 μmdiameter lenslets on 125 μm centers printed onto a glass substrate;

FIG. 16 is an optical micrograph of four 300 μm fibers, one with only apolished tip and three with printed lenslets;

FIG. 17 is an optical micrograph of a portion of an array of ellipticalplano-convex microlenses;

FIG. 18 is a schematic of an apparatus for agile beam steering;

FIG. 19 is a plot of beam steering angle versus decentered distance forthe apparatus shown schematically in FIG. 18;

FIG. 20 is a scanning electron microscope (SEM) micrograph of a portionof an array of hemi-elliptical microlenses; and

FIG. 21 is a schematic diagram of a process for fabricating compoundmicrolenses and cladded wave guides.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawing and FIGS. 1 and 2 in particular, shown thereinand generally designated by the reference character 10 is an ejectionhead or microjet for and capable of ejecting drops of optical materialfor forming micro-optical components. Ejection heads or microjets 10 arecapable of dispensing drops having a diameter as small as about 8 μm todrops having a diameter as large as about 300 μm. For purposes of thisapplication, the term "optical material" shall mean UV-curableadhesives, UV-curable resins, glasses, amorphous polytetrafluoroethylenesuch as Teflon® which is commercially available from E.I. DuPont deNemours, plastics, translucent polymers, solvenated polymers, polymerswith active materials such as dyes, sol-gel materials, optical waxes,optical epoxies, optical plastics and optical polymers. Preferably, theejection head 10 provides precise volumetric control and mostpreferably, such volumetric control is within ±2%.

Ejection head or microjet 10 comprises a first housing 12 ofpredetermined length and predetermined diameter. First end 14 of firsthousing 12 is formed with an orifice 18 ranging from about 8 μm to about300 μm in diameter. Second end 20 of first housing 12 is operativelyconnected to connector means 22. Connector means 22 is configured toallow tubing to be removably attached thereto for supplying liquidoptical material to first housing 12. It will be appreciated thataperture 24, which extends for the entire length of connector means 22,is operatively connected to the second end 20 of first housing 12. In apreferred embodiment, first housing 12 is tubular and is formed fromglass.

As shown in FIG. 2, a driver device 26 in the form of a housing ispositioned in operative contact around first housing 12. Driver device26 comprises an outer portion 28 and an inner portion 30, both of whichare formed of a metallic substance. Electrical leads 32 and 34 areoperatively connected to outer portion 28 and inner portion 30,respectively, and means 36 for electrically exciting or activatingcomprises a device for providing electrical pulses together withinterface electronics. Driver device 26 comprises any device forgenerating a pressure wave in first housing 12 to force a predeterminedamount of liquid optical material through the first housing 12 toorifice 18. Suitable driver devices 26 according to the presentinvention include transducers such as piezoelectric, electrostrictive,magnetostrictive or electromechanical transducers. Those of ordinaryskill in the art will recognize that while the driver device 26 is shownas a tubular element, the driver device 26 may be formed in any suitableshape.

According to certain embodiments of the present invention, the firsthousing 12 must be heated at temperatures up to 1100° to maintain theoptical material supplied to the housing 12 in a liquid state. In suchcases, the driver device 26 may be located outside the heating zone andcoupled to the ejection head 10 through mechanical couplers.

Second housing 38 has a first end 40 and a second end 42. Second housing38 is positioned in a surrounding coaxial relationship to first housing12 and is spaced therefrom to form a cavity 44 therebetween. First end40 is operatively attached to connector means 22 while second end 42 isoperatively attached to first end 14 of first housing 12 by adhesivematerial 46. Cavity 44 is filled with potting material 48 which iselectrically insulative. Electrical leads 32 and 34 pass throughapertures 50 and 52, respectively, in the wall of Second housing 38. Theprimary purpose of second housing 38 is to protect first housing 12 fromany external physical forces. In a preferred embodiment, second housing38 is tubular and is formed of a metallic or hard plastic material.Those of ordinary skill in the art will recognize that while the secondhousing 38 is shown as a tubular element, the second housing 38 may beformed in any suitable shape.

Further details concerning ejection heads and microjets such as thoseshown in FIGS. 1 and 2 may be found in U.S. Pat. No. 5,092,864, theentire disclosure of which is hereby incorporated by reference.

Referring to the drawings and FIG. 3 in particular, shown therein andgenerally designated by the reference character 60 is an opticalmaterial ejecting apparatus constructed in accordance with the presentinvention and which is capable of ejecting quantities of liquid opticalmaterial with great accuracy to very small areas or surfaces to bewetted.

As illustrated, the optical material ejecting apparatus 60 comprises anejection chamber 62 which includes heating element 64 operativelypositioned therein to maintain the optical material 66 in a liquidstate. Heating element 64 and thermocouple 68 are connected to powersource 70 so the optical material 66 will be maintained in a liquidstate. Heater 72 surrounds ejection device 74 and controls thetemperature of the liquid optical material 66 within the ejection device74. Heater 72 is connected to power source 76. Programmable controller78 provides activating signals to drive electronics 80 whose outputcauses ejection device 74 to eject a drop or drops 82 of liquid opticalmaterial 66 which is controlled by the particular program inprogrammable controller 78.

As shown in FIG. 3, the ejection device 74 comprises a drop-on-demandejection device employed to produce drops 82 of optical material 66. Thetarget substrate 84 could be moved by mounting target substrate 84 on anX-Y table 86 which is equipped with an X-axis positioning servo 88 and aY-axis positioning servo 90. Programmable controller 78 is operativelyconnected to the X-axis positioning servo 88 and the Y-axis positioningservo 90 and is structured to provide programmed control signals theretoto move the X-Y table 86 to a particular desired location and/or apredetermined sequence of locations with respect to ejection device 74.

A system for dispensing drops of liquid optical material for formingmicro-optical components utilizing an ejection head or microjet, such asthose shown in FIGS. 1-3, will be referred to herein as an "optics jetdroplet dispensing system". In an optics jet droplet dispensing system,a volumetric change in the liquid optical material within the ejectionhead 10 is induced by the application of a voltage pulse throughelectrical leads 32 and 34 to a driver device 26, preferably atransducer and most preferably a piezoelectric, electrostrictive,magnetostrictive or electromechanical transducer, which is directly orindirectly coupled to the liquid optical material. This volumetricchange causes pressure and velocity transients to occur in the liquidoptical material, and these are directed to produce a drop that issuesfrom the orifice 18 of the ejection head 10. In a preferred embodiment,the optics jet droplet dispensing system is a "drop-on-demand" system inwhich the voltage is applied only when a drop is desired. Adrop-on-demand system is to be distinguished from a "continuous" systemin which the optics jet droplet dispensing device produces a continuousstream of charged droplets which are directed to a target by deflectionplates. The drop-on-demand system of the present invention isdata-driven so that single droplets or bursts of droplets ejected atfrequencies up to 5 kHz may be printed at predetermined locations.

The system according to the present invention includes a microdropletgenerating device with the capability of dispensing optical materials inone embodiment at a temperature up to about 300° C. and in anotherembodiment at a temperature up to about 1100° C., controlling atmosphereand temperature during flight and controlling precisely the position(XY) of a target substrate. The system has the capability to dispensedroplets of optical materials from a drop-on-demand optics jet dropletdispensing device in one embodiment at a temperature up to about 300° C.and in another embodiment at a temperature up to about 1100° C., withsystems to control the temperature gradient along the droplet flightpath, and to heat or cool the target substrate in order to controlsurface wetting, solidification rate and curing processes.

According to the present invention, it has been determined that anoptics jet droplet dispensing system provided with means for dispensingoptical material at high temperatures and means for controlling targettemperature and surface condition, is a highly versatile tool forin-situ fabrication of micro-components and systems. Such an optics jetdroplet dispensing system enables direct printing onto opticalsubstrates or active devices of a wide range of commonly used opticalmaterials to create a variety of micro-optical components and systemsincluding high-density arrays of circular, cylindrical or squarelenslets, complex elements such as anamorphic lenslet elements,waveguides, couplers, mixers and switches, as well as monolithic lensesdeposited directly onto diode lasers facets or onto the tips of opticalfibers. For instance, and as shown in FIG. 4, an array of microlenses200 can be deposited directly on an array of diode lasers 202. As thoseof ordinary skill will recognize, the diode lasers 202 are conventionalin the art and include metal contacts 204, an N layer 206, preferablyformed of a doped silicon layer, a P layer 208, and insulating blocks210, preferably formed of silicon dioxide. The microlenses 200 depositedon the diode lasers 202 refract the output of the diode lasers 202 sothat more of the output from diode lasers 202 enters a correspondingfiber optic cable 212 than would enter the fiber optic cable 212 in theabsence of the microlenses 200.

Further details concerning optical material ejecting apparatus such asthat shown in FIG. 3 may be found in U.S. Pat. No. 5,229,016, the entiredisclosure of which is hereby incorporated by reference.

According to the method of the present invention, an optics jet dropletdispensing system utilizing an ejection head or microjet 10 such asdepicted in FIGS. 1 and 2, is used to produce droplets of opticalmaterials having a diameter ranging from about 8 μm to about 300 μm withprecise volumetric control, preferably within ±2%, and delivered withmicron accuracy to pre-selected target locations. According to thepresent invention, arrays of plano-convex lenslets, comprising opticalwax on glass substrates, have been produced with densities as high as10,000 per square centimeter.

The minimum feature size which may be printed by the optics jet dropletdispensing system of the present invention is determined by both thevolume of the smallest ejectable microdroplet, as controlled by theoptics jet droplet dispensing device orifice diameter, and the degree ofwettability of the substrate surface to the printed material. Single ormultiple droplets printed onto a flat substrate spread and coalesce toform plano-convex elements upon either or both of solidification andcuring. Hemispherical lenslets of differing diameters and speeds areformed by varying the number of droplets per substrate site, thesubstrate condition and either or both of the type and solidificationrate of the optical material used. Other types of micro-opticalstructures, such as cylindrical lenslets, microlens arrays and opticalinterconnects, may be printed by translating the substrate in the planeperpendicular to the optics jet droplet dispensing device andcontrolling its temperature.

The method of the present invention is data driven in that optics jetdroplet dispensing of optical material requires data in digital format.Accordingly, data can be used directly to control the jetting whichresults in flexible manufacturing platforms. For instance, the methodcan be used to directly place or mount differently sized and configuredoptical circuit patterns onto a single substrate plane or onto the endsof optical fibers in an array or bundle, with the capability forreal-time, design changes. Also, and as shown in FIG. 5, an optics jetdispensing system 310 of the present invention can be combined with avision system 312 that includes focusing optics 314 so as to correct forsystem assembly errors. In such an arrangement, a fiber optic holder 316holding a fiber optic bundle 318 may be located on an X-Y alignmentstage 320. The X-Y alignment stage 320 can be manipulated to allow thevision system 312 to detect the precise location of the end of eachfiber in the fiber optic bundle 318 and then the X-Y alignment stage 320can be manipulated again so that the fiber optic bundle 318 is locatedbelow the optics jet dispensing system 310 which can then preciselydispense drops 322 of liquid optical material so that the drops land onthe ends of the fibers in the fiber optic bundle 318, the location ofwhich is identified by the vision system 312. The method can also beused to directly generate mask patterns for construction ofsemiconductor micro-optics by subsequent dry etching or ion-beammachining processes.

According to the present invention, an optics jet dispensing system canbe constructed with multiple jets which would allow for the printing onthe same substrate of more than one optical materials such as active andpassive materials or materials with different refractive indices.

Also according to the present invention, the size of the droplets ofoptical material issuing from the aperture of the ejection head can bevaried. The size of the droplets can be varied by changing the data usedto control the jetting of the optical material. For instance and asshown in FIG. 6, multiple ejection heads 410 are provided for dispensingdrops of optical material onto a substrate 412. The substrate 412 isformed of a material that is easily wetted with the optical materialbeing dispensed from the ejection heads 410. Prior to dispensing thedrops of optical material, the substrate 412 may be masked with anon-wetting coating 414 such that only open circles of the easily wettedsubstrate 412 are exposed. When a drop of optical material is dispensedonto the exposed circular easily wetted portions of the substrate 412,the drop spreads out to cover the area but does not coat the non-wettingcoating 414. FIG. 6 demonstrates two methods for increasing the amountof optical material dispensed onto a particular exposed circular easilywetted portion of the substrate 412. First, larger or smaller dropletscan be generated by varying the shape and/or amplitude of the electricalpulse that drives the ejection devices 410. As shown in FIG. 6, a largedrop 416 forms a lens shape 418 with a greater curvature and shorterfocal length when compared to a smaller drop 420 and the resultant lensshape 422. Second, multiple drops 424 can be directed to the sameexposed circular easily wetted portion of the substrate 412 to create alarger amount of fluid deposited, thus forming a lens shape 426 with agreater curvature and shorter focal length when compared to the lensshape 428 formed by fewer drops 430.

Also, the curvature and focal length of the deposited lens shapes can bealtered by dispensing from the ejection devices multiple fluids havingdifferent characteristics. Thus, by varying the size of the droplets,the number of droplets, and by varying the contour of the lens surfacedepending on the contact angle between the droplet and the substrate andthe wetting of the substrate, microlenses with variable focal length canbe produced.

According to the present invention, the optics jet droplet dispensingsystem can be used to create three dimensional structures of high aspectratio by delivering multiple droplets of optical material to a targetlocation. By varying the number of droplets dispensed and the scanningpattern, a wide variety of optical lenslet configurations can be createdat different locations on the same substrate or system. According to thepresent invention and as shown in FIGS. 7 and 8, an optics jetdispensing system can be used to create three dimensional verticalstructures 102 and 104, respectively, by delivering multiple droplets,shown schematically as 106, to a substrate 108.

According to the present invention and as shown in FIG. 9, an optics jetdispensing system can be used to create three dimensional horizontalstructures such as waveguides 110 and 112, by delivering multipledroplets, shown schematically as 114, to a substrate 116. The waveguides110 and 112 receive light input 118 and 120. The light input 118entering waveguide 110 passes through electrodes 122 and 124 whichmodulate the light input 118. After passing through the electrodes 122and 124, the light input 118 is combined with light input 120 to producelight output 126.

According to the present invention, various optical materials can bedispensed as microdroplets so long as they can be formulated by heatingand/or mixing with a solvent to have a viscosity of less than 40centipoise in the ejection head of the optics jet droplet dispensingsystem. For instance, according to the present invention, an optics jetdroplet dispensing system can be used to dispense optical materials suchas waxes, greases, epoxies, various polymers including amorphousTeflon®, polyimide and photoresist and sol-gel materials at temperaturesas high as 300° C. Also, by removing the driver device from the heatingzone, an optics jet droplet dispensing system can be used to dispenseoptical materials at temperatures up to 1100° C.

According to a preferred embodiment of the present invention, theformulations of optical materials for optics jet droplet printing ofmicro-optics satisfy two requirements. Namely, after solidification andcuring they provide both the optical and physical properties needed forthe particular application. And they are reducible in viscosity belowabout 40 centipoise, by either heating or modifying the polymerchemistry by additions of diluents or solvents, so as to be readilyprintable by the method of the present invention. Solvent-free materialsat elevated temperatures are preferred according to the method of thepresent invention since evaporation of solvent from microjetted dropletsduring solidification may cause size and shape distortion in thefinished product.

According to a preferred embodiment of the present invention, the typesof micro-optical materials to be dispensed as microdroplets includeindex-tuned thermoplastics and hydrocarbon resins dispensed at 100°-200°C., along with UV-curing optical adhesives microjetted at roomtemperature.

According to the present invention, optical materials and the targetsubstrate can be processed before, during and after delivery of theoptical material to result in products having different characteristics.The pre-delivery processing techniques include heating, melting anddissolving the optical material as well as substrate preparations suchas etching or polishing. The in-flight processing techniques includecooling, mixing and curing. The post-deposition processing techniquesinclude heat treatment, curing, chemistry and surface finishing. Forinstance, as shown in FIG. 10, an optics jet droplet dispensing system510 can be combined with a source of radiation 512, such as a UV lightsource or a laser beam, and the source of radiation 512 can be alignedwith the optics jet droplet dispensing system 510 to cure the opticalmaterial after forming a microlens shape 514 on a substrate 516. Theimpact of a droplet of optical material 518 on the substrate 516 can bealigned in time and space with such a source of radiation 512. Theforegoing curing technique is applicable to micro-optical elements otherthan microlenses.

The numerical apertures of lenslets formed by microdroplets of a givenvolume of optical material deposited onto a substrate depend upon itssurface contact angle, the viscosity of the optical material, the rateof solidification of the optical material and the materials surroundingthe site of deposition of the optical material on the substrate. Theviscosity of the droplets of optical material at impact can be adjustedby varying the dispensing temperature, the cooldown rate before andafter impact, for instance via an optical energy source such as UV lightin the flight chamber and/or an IR laser directed at the target site,and substrate surface condition, such as wettability.

According to the method of the present invention, multiple ejectionheads or microjets, such as those shown in FIGS. 1 and 2, can be used tosimultaneously microdispense different materials such as active andpassive materials with or without different indices of refraction. Also,multiple ejection heads or microjets with different orifice sizes can beused to simultaneously dispense different sized droplets of opticalmaterial. Finally, multiple ejection heads or microjets can be used tomix different materials, such as the components of a compound opticalmaterial, in flight or at the substrate target.

In order to be dispensed from an ejection head or microjet according tothe present invention, an optical material must have a viscosity lessthan about 40 centipoise as it exits the orifice of the ejection head ormicrojet. The temperature of optical materials having a higher pre-cureviscosity may be elevated to bring their viscosities into an acceptablerange for dispensing from an ejection head. Table I lists various typesof materials which are of interest for microoptics, along with theapproximate temperature to which each must be raised to achieve therange of viscosity needed for dispensing as microdroplets.

                  TABLE I                                                         ______________________________________                                        Optical Materials for Microdroplet Dispensing, with                           approximate Dispensing Temperature                                                                 Dispensing Temperature                                   Material Type        (°C.)                                             ______________________________________                                        Polymers (photoresist, Teflon ®)                                                               <100                                                     Sol-gels             <50                                                      Optical waxes        <120                                                     Optical epoxies      80-100                                                   UV-curable resins & adhesives                                                                      100-150                                                  Polymers with active materials (dyes)                                                              <100                                                     Optical plastics     100-500                                                  Optical glasses      600-1100                                                 ______________________________________                                    

According to the method of the present invention, optical wax at 100° C.was jetted onto a glass slide to form an array of 80 μm plano-convexlenslets on 100 μm centers.

The suitability of an optical material for use in the method of thepresent invention is assessed according to the following protocol:

(a) Viscosity data as a function of temperature to determine the optimaltemperature for dispensing droplets of an optical material is obtained;

(b) The optimum drive waveform, voltage, and frequency formicrodispensing an optical material from an optics jet dropletdispensing system is determined and single droplets and bursts ofmultiple droplets are dispensed at different locations on opticalsubstrates for testing and evaluation;

(c) Electron and optical microscopy is used to inspect the depositeddroplets of dispensed material for defects such as second phases andmicrocracks, and to measure their dimensional characteristics;

(d) The dispensing parameters are varied such as in-flight cool downrate, dispenser device temperature, substrate temperature, and firingfrequency (for droplet bursts) to optimize the uniformity of thedeposited material;

(e) The visible light transmission and focal length ranges of the mostuniform deposits are measured, using a calibrated detector and beamprofiler to enable quantitative comparisons among various opticalmaterials;

(f) Compare the lenslet formed from an optical material to lensletsformed from other optical materials based on considerations such asmaximum refractive index, coupling efficiencies and numerical apertures,along with minimum lens dimensions.

According to the present invention, the ranges in lenslet andlenslet-array design parameters can be determined without impactingadversely the optical quality or efficiency of the lenses bymanipulation of the microdroplet deposition process parameters. As shownin FIG. 11, the focal lengths and numerical apertures of plano-convexlenslets formed by droplets of a given optical material that isdeposited onto a substrate can be varied by changing such parameters asthe number of droplets per site, the viscosity of the droplet material,as well as the condition, treatment or temperature of the substratesurface. Therefore, the numerical apertures of a lenslet formed fromdroplets from an optics jet droplet dispensing device with a certainorifice diameter can be increased by reducing the wettability of thesubstrate surface, increasing the viscosity of the droplet at the momentof impact (for instance by cooling it in flight) or by depositing one ormore additional droplets on the top of the first droplet after it ispartially solidified.

Specifically, FIG. 11(a) is a schematic representation of the profile ofa lenslet produced under the following process parameters: singledroplet, low viscosity, hot substrate and high-wetting surface. FIG.11(b) is a schematic representation of the profile of a lenslet producedunder the following process parameters: single droplet, higherviscosity, cool substrate, and low-wetting surface. FIG. 11(c) is aschematic representation of the profile of a lenslet produced under thefollowing process parameters: multiple droplets, cool substrate, andlow-wetting surface. The lenslets shown in FIG. 11 are made from thesame optical material and the lenslet of FIG. 11(a) has a lowernumerical aperture than the lenslet of FIG. 11(b) which in turn has alower numerical aperture than the lenslet shown in FIG. 11(c).

According to the present invention, the effectiveness of the jettedmicrolenses has also been determined for use in very fast lensapplications such as collimating light emitted from the narrow apertureof a diode laser or an optical fiber. For free space operation, thefocal length of a plano-convex spherical lens with radius R made from amaterial with an index of refraction n is f=R/(n-1). For light ofwavelength λ diffracting from an aperture of diameter a, the collimatingradius of curvature R_(c) is found from ray tracing through a thick lensof approximate height h. Using geometrical considerations, R_(c) may berelated to the height h and the lens diameter c. Thus, the aperture is afunction of h and c, which constitute lens fabrication parameters andover which there is precise control. Specifically, ##EQU1##

A moderate requirement is that c be greater than a. This equation isplotted as an engineering contour plot in FIG. 12 for n=1.3 and awavelength of 500 nm. This plot can be interpreted to find the contourcorresponding to a given aperture. The axes then give the range ofspecifications for the collimating lens. The parameters measured forseveral optical wax lenslet arrays that were made and tested accordingto the present invention are marked with X's on the plot and showapertures in the 9-15 μm range, which indicates that collimatingapertures in the 5-100 μm range can be made by the process of thepresent invention using practical values of c and h. These aperturedimensions correspond to single and multimode fibers, and to narrow andwide stripe diode lasers and amplifiers.

The ranges in focal length .and numerical apertures which can beobtained with nearly spherical plano-convex lenslets made using theoptics jet droplet dispensing process for a particular optical materialcan be determined by:

(a) increasing the number of droplets deposited at the same site andallowing each droplet to solidify at its substrate-contacting surfacebefore placing the next drop on top of it; and

(b) decreasing the wettability of the substrate deposition site prior toplacement of the first droplet by, for example, polishing or cleaningthe entire substrate surface, or roughening by laser etching only thesites to be covered by the lenslets.

The varieties of useful configurations of nearly cylindrical andanamorphic microlenslets for use in focusing or collimating, shaping andsteering beams, for example, in diode laser and amplifier systems, canbe determined by:

(a) translating the substrate stage during droplet deposition;

(b) coordinating droplet emission frequency with substrate speed; and

(c) controlling substrate surface condition, temperature and coolingrate.

The highest densities of arrays of microlenses with properties which canbe used for diode laser array coupling applications can be determinedby:

(a) ejecting droplets of optical fluids less than 25 microns in diameterby reducing the microjet orifice diameter to less than 20 microns; and

(b) varying substrate surface wettability and/or viscosity of dispensedformulations to obtain lenslets with shallow features and low numericalapertures for diffraction, deep features and high numerical aperturesfor refraction applications.

Scanning electron microscope (SEM) studies and optical characterizationmeasurements can be performed on the above lenslet configurations todetermine the values of key performance parameters achieved by theprocess of the present invention to determine the best lenslets forincorporation into optical systems and substrates.

The process of the present invention can be used to deposit focusing orcollimating lenslets onto the outlet facets of both edge-emitting andsurface-emitting diode lasers.

The advantages of the method of the present invention for making andmounting microlenses for collecting, collimating and focusing diodelaser output beams include:

(a) the capability to deposit lenslets directly onto the emitting facetof a diode, thereby providing greater control over beam divergence thanproximity lenslets; and

(b) lower production cost and greater flexibility in lens formationcompared to thin film deposition and etching methods.

According to this process, lenslets are deposited onto test diode lasersand then the diode-lenslet systems are tested for optical performance.

A schematic diagram of a micro-optics printing station according to thepresent invention is generally designated in FIG. 13 by the referencenumeral 600. A microjet printhead 602 having a 5 μm mesh filter,preferably formed of stainless steel, is contained within a heatingshell 604 which is connected to a heated fluid reservoir 606. A computer608 controls a function generator 610 which in turn produces theprinthead drive waveform and the motion of the XY-stage 612 which isdriven by the X-Y stage driver 614. The computer also sets all of themicrojet printing parameters. A pulse generator 616, triggered by thefunction generator 610, drives an LED 618 positioned below the orificeof the printhead 602 to provide stroboscopic illumination of thesuperimposed images of the ejected microdroplets during pre-printingmicrojetting parameter optimization. The superimposed images areviewable with a vision system 620 comprising a microscope 622, a camera624 and a television 626. The temperatures of the fluid reservoir 606,printhead 602 and substrate 628 are independently regulated by thetemperature controller 630 to control fluid viscosity at the orifice ofthe printhead 602 and printed microdroplet solidification rate.

The micro-optics printing process according to the apparatus shown inFIG. 13 involves: (i) adjusting the temperature of the printhead 602 andthe parameters of the drive waveform to achieve stable microdropletformation with the particular optical material to be printed, (ii)setting the surface condition and temperature of the substrate 628 foroptimal micro-optical element formation, then (iii) specifying thespecific pattern of print sites and the number of microdroplets to beprinted per site. According to this process, cleaning of the substrate628 is an important step in the process and the surface of the substrate628 may or may not be treated with a low-wetting coating to inhibit orencourage, respectively, droplet spreading and coalescence prior tosolidification.

Plano-convex spherical microlenses can be fabricated according to themethod of the present invention by microjetting microdroplets ofmaterials such as optical adhesive material at room temperature oroptical thermoplastic formulations at elevated temperatures ontosubstrates such as glass slides, silicon wafers and the tips of opticalfibers, and then solidifying the droplets by UV-curing or cooling,respectively. According to this process, the diameter of the lenslets isdetermined primarily by the size and number of microdroplets deposited,and the speed of a microlens of a given diameter depends on the dropletsurface tension and the wettability of the substrate to the materialbeing deposited. This is illustrated in FIG. 14 which is a plot of focallength versus lenslet diameter for microlenses of low-index (n_(D)=1.53) optical adhesive, high-index (n_(D) =1.704) thermoplasticmaterial and high melting point resin, dispensed at 25° C., 145° C. and200° C., respectively. All of the lenslets were printed onto virgin(untreated) glass slides except for one set of the solvent-bearingadhesive microlenses wherein the glass was pre-treated for low wetting(designated as "ADHESIVE-L.W.G."). In all cases and as shown in FIG. 14,focal length varies linearly with diameter, while lenslet speed, asdetermined by line slope varies independently of diameter with lensletmaterial and substrate surface condition. Lenslet speed varies by nearlya factor of two over the range f/1.5 to f/2.6 by changing themicrojetted material and substrate surface preparation. For example, a30% increase in speed can be obtained with the optical adhesive byreducing the wettability of the glass substrate.

According to the method of the present invention a 50×50 array of 107 μmdiameter lenslets on 125 μm centers was printed onto a glass substratetreated with a silicone formulation for low surface wetting, bydispensing 50 μm droplets of a 2:1 mixture of Norland Products #71optical adhesive and acetone at room temperature and then curing thestructure under UV illumination. An optical micrograph of a portion ofthe array is shown in FIG. 15. Microscopic measurements at 100X oflenslets selected at random over the entire array indicated a standarddeviation in diameter of 2 μm. A far-field diffraction pattern underback-illumination with collimated light showed good uniformity andclarity which indicated good lenslet formation, optical quality andplacement for the array.

According to the method of the present invention, hemispherical lensletswere printed directly onto the polished tips of optical fibers. Whenprinting lenslets onto such optical fibers, the diameter of the lensletsis constrained by the fiber width but the radius of curvature of thelenslets may be tailored over a wide range by varying the number ofdeposited microdroplets. For instance, a lenslet radius of curvaturerange of 150 to 300 μm was achieved by depositing onto the polished tipof a 300 μm fiber by increasing the number of 57 μm deposited dropletsfrom 100 to 200. FIG. 16 is an optical micrograph of four 300 μm fibers,one with only a polished tip and three with printed lenslets. Accordingto the method of the present invention, lenslets can be printed directlyonto the unpolished tips of optical fibers when the polymer formulationshave an index of refraction that matches the index of refraction of theoptical fiber.

According to the method of the present invention, plano-convexmicrolenses have been fabricated from thermoplastic formulations byplacing adjacent microdroplets at spacings which enable them to coalesceinto a single elliptical droplet prior to solidification. Specifically,each lenslet in a 20×30 array of cylindrical microlenses was formed bymicrojetting four 50 μm droplets of 1.704 index thermoplastic at 145° C.onto a glass substrate maintained at 45° C., at droplet to dropletspacings of 75 μm. An optical micrograph of a portion of the array isshown in FIG. 17. The microlenses had a size of 304 μm×196 μm and werevery reproducible, varying in major and minor axis dimension over theentire array by standard deviations of less than 4 μm. The overall sizeand length/width aspect ratio of the elliptical lenslets, along withplacement positions, can be tailored to match a particular applicationor to add value to an electro-optical device. For instance, theelliptical lenslets were used to increase the efficiency of coupling theoutput of edge-emitting diode laser arrays to an optical fiber or theend of a solid-state laser rod, by varying the printing processparameters. The regularity and uniformity of the far-field diffractionpattern of such an array attested to the overall good optical quality ofthe array.

According to the process of the present invention, ten channelwaveguides having a ridge configuration with a cylindrical cross-sectionwere microjet printed. These waveguides can be used in applications suchas multimode board-to-board, backplane interconnects. Specifically, anarray of 18 mm long and 134 μm wide cylindrical waveguides of high-index(n_(D) =1.704) thermoplastic was formed on glass slides (n_(D) =1.53).Each waveguide was formed by dispensing 250 each 50 μm droplets at 145°C. onto a 45° C. glass substrate on 75 μm centers, where they coalescedprior to solidification to form very uniform and straight lines. Theprocess for printing the waveguides was similar to the process forfabricating elliptical microlenses and gave a standard deviation inwidth over the entire length all ten waveguides of about 2 μm.

The small sizes and short focal lengths achievable with microlenses canbe utilized for agile beam steering, because large angular changes inbeam propagation direction can arise from very small displacements inlenslet position. To demonstrate this property of micro-optics, atelescope was fabricated with micro-jet printed refractive microlensarrays, using the apparatus shown schematically as 700 in FIG. 18, andscan angle as a function of decentering of a first array 702 relative toa second array 704 was measured. A beam 706 from a HeNe laser 708 wasexpanded by a factor of 30 by a beam expander 710 and recollimated usinga standard Galilean telescope 712. A micropositioner 714 was used to setthe spacing of the two arrays 702 and 704, oriented face-to-face, equalto the sum of their focal lengths and to translate the second array 704relative to the first array 702 in the transverse plane. Steeringagility was obtained by measuring the displacement of the center of thefar-field pattern 716 as a function of array translation.

The arrays 702 and 704 consisted of 20×20 spherical plano-convexlenslets, printed with UV-curing optical adhesive on 700 μm centers ontolow-wetting glass substrates. The lenses of the first array 702 in theoptical path were 530 μm in diameter and had a focal length of 720 μm.Two arrays 702 and 704 were used in the scanner position having lensletdiameter and focal length of 530 μm and 800 μm; and 440 μm and 540 μm,respectively. The data for these two configurations are shown in FIG. 19which is a plot of beam steering angle versus array decenter distance.The data followed the expected governing relationship among the scanangle μ, the focal length f2 of the decentered array and the decenterdistance Δ where Δ is determined according to the following formula:tan(μ)=Δ/f2. As shown by FIG. 19, an array of fast (f/1.2) 500 μm focallength lenses decentered by only 100 μm produced a scan angle of 11degrees and a total scan of about 40 degrees was achieved with a nearlylinear relationship between steering angle and array decenter.

The methods of the present invention are quite advantageous for certainapplications in that they provide low cost (no photolithographic masksand minimal usage of optical materials), manufacturing flexibility(data-driven), in situ, non-planar processing (direct writing,non-contact) and the ability to create new micro-optical structures suchas arrays of lenslets of differing shapes and speeds in the same array.

According to the method of the present invention, plano-convex,hemi-elliptical microlenses were fabricated by the microdispensing ofindividual microdroplets of an optical thermoplastic at a temperaturesubstantially above the melting point of the material onto a clean,defect-free optical substrate maintained at a temperature below themelting point, in such a fashion that the droplets partially coalesceimmediately before they solidify by cooling to the substratetemperature. The size and shape (major and minor axes and theirrespective radii of curvature) of the microlenses can be varied bychanging such parameters as: material formulation, droplet size, numberof droplets deposited, droplet spacing, temperature differential betweenmicrojetted material and substrate and the substrate surfacewettability. FIG. 20 shows a scanning electron microscope (SEM)micrograph of a portion of an array of microjet printed hemi-ellipticalmicrolenses that were fabricated by microjetting four overlappingdroplets of an optical thermoplastic onto a glass substrate. Themicrolenses have a major/minor axis ratio of 1.7 and a radius ofcurvature along the major axis of about 330 μm.

The plano-convex refractive microlenses having the shape of ahemi-ellipsoid of revolution that were prepared according to the methodof the present invention, are capable of collecting the anisotropicallydiverging light from an edge-emitting diode laser to bring it to asingle focal point, for coupling into an optical fiber or the end of asolid state laser rod. An array of such microlenses, appropriately sizedand positioned, is of great benefit in increasing the efficiency ofcoupling or arrays of power diode lasers to optical fibers or laserrods, particularly since the microlenses can be fabricated in situ andcustomized by changing the input data, in order to optimize the couplingperformance of each individual system.

According to the method of the present invention, plano-convexrectangular microlenses with uniformly domed tops were fabricatedaccording to the same general method as described above for ellipticalmicrolenses, except that the microdroplets for each lenslet weredeposited on the target substrate as two dimensional arrays instead ofalong a line. The size and shape of the microlenses are determined bythe numbers of overlapping microdroplets deposited along the twoperpendicular directions in the substrate plane along with the otherprocess control parameters set forth above with respect to theelliptical microlenses.

The plano-convex microlenses produced according to the presentinvention, have a rectangular shape and a uniformly domed top and areuseful in high fill-factor stand alone arrays to maximize lightcollection efficiency from a variety of sources. Such microlenses areideally suited for maximizing the efficiency of light collection by CCDarrays, as the microlenses can be appropriately sized and formed on eachindividual pixel surface.

According to another method of the present invention, compoundmicrolenses and cladded multimode waveguides can be fabricated toimprove optical performance. As shown in FIG. 21, the process forfabricating a compound microlens array shown generally at 800 involvesprinting on an optical substrate 806 which in turn is disposed over atranslatable temperature controlled XY stage 808, a desired pattern ofmicro-optical elements using, for example, a high index thermoplasticformulation in a first jet 802 and then repeating the identical patternwith a lower index formulation in a second jet 804. To print claddedwaveguides, the XY stage would be translated normal to the plane of FIG.21 during each deposition step in order to microdeposit adjacent,coalescing droplets along the desired waveguide direction, therebycreating ridge channel waveguides of hemi-cylindrical cross-section. Thecritical parameters for fabricating compound microlenses and claddedmultimode waveguides include the use of optimal amounts of compatibleoptical materials for the micro-optical elements and the maintenance ofprecise substrate registration relative to the printhead during theprocess.

While the present invention has been described with reference to apresently preferred embodiment, it will be appreciated by those ofordinary skill in the art that various modifications, changes,alternatives and variations may be made therein without departing fromthe spirit and scope thereof as defined in the appended claims.

What is claimed is:
 1. A method for producing micro-optical components,comprising the steps of:(a) maintaining an optical element formingmaterial in a liquid state in an ejection chamber; (b) transferring saidliquid optical element forming material from said ejection chamber to anejection device; (c) maintaining said optical element forming materialin a liquid state in said ejection device, said optical element formingmaterial having a viscosity of less than 20 centipoise at a temperatureof from 100° to 200° C. in said ejection device; and (d) ejecting dropsof said liquid optical element forming material from said ejectiondevice to a surface to be wetted.
 2. A method for producingmicro-optical components, comprising the steps of:(a) maintaining anoptical element forming material in a liquid state in an ejectionchamber; (b) transferring said liquid optical element forming materialfrom said ejection chamber to an ejection device; and (c) ejecting dropsof said liquid optical element forming material from said ejectiondevice to a surface to be wettedwherein said surface to be wettedcomprises an end of an optical fiber, wherein said optical fiber has adiameter of from 50 to 300 μm and wherein multiple drops of said opticalelement forming material are deposited on said end of said optical fiberto produce a microlens having a radius of curvature that varies with thenumber of drops of said optical element forming material deposited.
 3. Amethod for producing micro-optical components, comprising the stepsof:(a) maintaining an optical element forming material in a liquid statein an ejection chamber; (b) transferring said liquid optical elementforming material from said ejection chamber to an election device; and(c) ejecting drops of said liquid optical element forming material fromsaid election device to a surface to be wetted, wherein saidmicro-optical components are plano-convex microlenses and whereinadjacent drops of said liquid optical element forming material ejectedfrom said ejection device to said surface to be wetted coalesce into asingle elliptical droplet prior to solidification to form a plano-convexhemi-elliptical microlens.
 4. A method according to claim 3 wherein theshape of said plano-convex, hemi-elliptical microlens may be adjusted byadjusting the formulation of said optical element forming material, thesize of said drops, the number of drops deposited, the spacing of saiddrop on said surface to be wetted, the temperature differential betweensaid ejected optical element forming material and said surface to bewetted, and the wettability of said surface to be wetted.